Xlib generally calls the program's supplied
error handler whenever an error is received. It is not called on BadName
errors from OpenFont, LookupColor, or AllocNamedColor protocol
requests or on BadFont errors from a QueryFont protocol request. These
errors generally are reflected back to the program through the procedural
interface. Because this condition is not assumed to be fatal, it is acceptable
for your error handler to return; the returned value is ignored. However,
the error handler should not call any functions (directly or indirectly)
on the display that will generate protocol requests or that will look for
input events. The previous error handler is returned.

The XGetErrorText
function copies a null-terminated string describing the specified error
code into the specified buffer. The returned text is in the encoding of
the current locale. It is recommended that you use this function to obtain
an error description because extensions to Xlib may define their own error
codes and error strings.

The XDisplayName function returns the name of
the display that XOpenDisplay would attempt to use. If a NULL string
is specified, XDisplayName looks in the environment for the display and
returns the display name that XOpenDisplay would attempt to use. This
makes it easier to report to the user precisely which display the program
attempted to open when the initial connection attempt failed.

The XSetIOErrorHandler
sets the fatal I/O error handler. Xlib calls the program's supplied error
handler if any sort of system call error occurs (for example, the connection
to the server was lost). This is assumed to be a fatal condition, and the
called routine should not return. If the I/O error handler does return,
the client process exits.

Note that the previous error handler is returned.

The XGetErrorDatabaseText function returns a null-terminated message (or
the default message) from the error message database. Xlib uses this function
internally to look up its error messages. The text in the default_string
argument is assumed to be in the encoding of the current locale, and the
text stored in the buffer_return argument is in the encoding of the current
locale.

The name argument should generally be the name of your application.
The message argument should indicate which type of error message you want.
If the name and message are not in the Host Portable Character Encoding,
the result is implementation-dependent. Xlib uses three predefined ``application
names'' to report errors. In these names, uppercase and lowercase matter.

XProtoError

The
protocol error number is used as a string for the message argument.

XlibMessage

These
are the message strings that are used internally by the library.

XRequest

For
a core protocol request, the major request protocol number is used for
the message argument. For an extension request, the extension name (as given
by InitExtension) followed by a period (.) and the minor request protocol
number is used for the message argument. If no string is found in the error
database, the default_string is returned to the buffer argument.